In integrated circuit systems, there are typically a number of circuits requiring DC voltages exceeding the supply voltage for fulfilling particular tasks. For example, in a flash memory device, a high voltage must be generated for data programming and erasing. A flash memory device incorporates several arrays of memory cells each typically resembling a field-effect transistor (FET) including a floating gate residing on the surface of a tunneling oxide layer for accumulating a charge corresponding to one bit of data information. In the flash memory device, data programming and erasing is accomplished by performing such control that a charge is injected in or released from the floating gate. The data programming in the flash memory device requires injection of a charge from a channel into the floating gate via the tunneling oxide layer by means of hot-electron injection. In doing this, the hot-electron injection requires high energy to make the charge pass through the gate oxide. The erase of data from the flash memory device requires the charge in the floating gate to be drawn back to the channel by tunneling through the tunneling oxide layer. During this process, the tunneling requires even higher energy to enable the charge to travel back to the channel. For a memory device, data programming typically requires a voltage that is far higher than its supply voltage. Conventional flash memory devices are powered by a voltage of 1.5 V and require a voltage in the range of from 7 V to 8 V for enabling their data programming. For this reason, conventional flash memory devices incorporate a charge pump in their circuitry to raise the 1.5 V power supply voltage to 7-8 V that is required by the data programming operation.
Conventionally, the charge pump is constructed by series-connected stages of boost capacitor circuits each having a boost capacitor for accumulating a charge and thereby driving an input voltage to a higher output voltage. However, in the conventional charge pump, the boost capacitor circuits operate with voltage amplitude equal to the working voltage VDD and can accumulate a boost charge C×VDD on each unit area of the capacitors, where C represents capacitance per unit area. C×VDD measures area efficiency (i.e., the amount of charge that can be provided by each unit area of the capacitors) of the charge pump. Therefore, the conventional charge pump can accumulate a low amount of charge on each unit area, i.e., low area efficiency.